put the update button code into it's own file

src/util/InitUpdateButton.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <memory>
+
+namespace pajlada {
+namespace Signals {
+class SignalHolder;
+}
+}  // namespace pajlada
+
+namespace chatterino {
+
+class RippleEffectButton;
+class UpdatePromptDialog;
+
+void initUpdateButton(RippleEffectButton &button, std::unique_ptr<UpdatePromptDialog> &handle,
+                      pajlada::Signals::SignalHolder &signalHolder);
+
+}  // namespace chatterino